Etude des instabilités volumiques en extrusion de polystyrène et polypropylène

Abstract : We experimentally investigated the helical and chaotic instabilities of two polystyrenes and a polypropylene, with two extrusion devices: a capillary rheometer and a transparent slit die fed by an extruder, where birefringence experiments and laser-Doppler velocimetry measurements are feasible. In the last case, the flow geometry consists in a reservoir, followed by a contraction and a slit. In capillary rheometry, we focused on the flow conditions influence (diameter, lenght, entry angle die, temperature...) on the helical instability onset. In birefringence experiments, we observed isochromatic fringe oscillations in the reservoir: in this unsteady regime, the main flow reaches the contraction asymmetrically. The use of a smooth entry convergent permits to postpone the defect occurrence to flow rates seven times higher than with an abrupt contraction. Moreover, we measured, on the reservoir axis, the first normal stress difference and the elongation rates in critical flow conditions, by flow birefringence and laser-Doppler velocimetry, respectively. Finite elements simulations, using a viscoelasticity model, were performed to calculate elongational stresses around the contraction, for critical flow conditions relative to instability onset.
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  • HAL Id : tel-00413026, version 1

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Christelle Combeaud. Etude des instabilités volumiques en extrusion de polystyrène et polypropylène. Mécanique [physics.med-ph]. École Nationale Supérieure des Mines de Paris, 2004. Français. ⟨tel-00413026⟩

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